Recently, with the spread of electric home appliances equipped with an infrared radiation type wireless remote control (hereinafter, referred to as infrared remote control), there has arisen a problem that products may misidentify other manufacturers' signals or signals derived from other products, resulting in malfunctions. Due to this, the Association for Electric Home Appliances (AEHA) intends to prevent the occurrence of such malfunctions by prescribing a data transmitting signal format for the infrared remote control. The following describes the construction of this signal format defined by the Association for Electric Home Appliances.
FIG. 2 shows the construction of the signal format. As shown in this figure, a signal sequence consists mainly of a leader L indicating the start of the signal sequence, a custom code C(C0, C1) identifying the company that supplies the product, a parity P, a data code D(D1–Dn) indicating specific information to be conveyed and a trailer TR indicating the end of the signal sequence. These are transmitted in this order to execute one control command. Of these signals, FIGS. 3A–3C show the shapes of standardized major pulse signals. That is, FIG. 3A shows the shape of the pulse signal in the leader portion L, FIG. 3B shows the shape of the pulse signal in the data code portion D and FIG. 3C shows the shape of the pulse signal in the trailer portion TR. These signals are constituted by pulses whose widths are integer multiples of a fundamental signal length T. As shown in the figures, a substantially rectangular pulse signal composed of H and L levels is designed to make up specific data depending on the H level pulse width and the L level pulse width so that what signal has been transmitted and received can be discriminated by detecting these pulse widths. Specifically, if a signal is transmitted whose ratio of the H level pulse width to the L level pulse width is 8T: 4T as shown in FIG. 3A, this signal is decided as a leader L indicating the start of the signal sequence. If a signal is transmitted whose H level pulse width is T and whose L level pulse width is 8 ms or longer as shown in FIG. 3C, this signal is decided as a trailer TR indicating the end of the signal sequence.
Meanwhile, the shapes of pulses in the data code portion D indicating the information are dependent on how “0” and “1” data are combined as shown in FIG. 3B. Each of the data is distinguished from the other by H level pulse width and L level pulse width. That is, if the ratio of the H level pulse width to the L level pulse width is T:T (1:1), the received data is recognized as “0” data. If the ratio of the H level pulse width to the L level pulse width is T:3T (1:3), the received data is recognized as “1” data. It is also prescribed that the ratio of the sum of the respective level pulse widths of “0” data to that of “1” data should be 1:2. Further, the fundamental signal length T is prescribed to fall within in the range of T=350 μs–500 μs, 3T=1050 μs–1500 μs).
In this connection, there has been a problem that if the pulse signal of the data code portion D is transmitted as it is in the exact waveform prescribed by the signal format, communication errors tend to occur because the waveform on the reception side is unsharpened on the whole as shown in FIG. 5 depending on the infrared photodetector's characteristics, noise-reducing capacitor and so on, resulting in shifts in the ON/OFF timing of the waveform. More specifically, the waveform recognized by a microcomputer on the reception side is affected by threshold settings, with a tendency that H level pulse widths on the transmission side are recognized longer and L level pulse widths shorter. Therefore, the waveform on the reception side may not satisfy the prescribed requirements of the range of fundamental signal length, T=350 μs–500 μs, and the ratio among the individual pulse widths, as a problem.
It is conceivable to adopt a method in which, as shown in FIGS. 6A–6D, L level pulse widths of a transmission-side waveform are set longer than its H level pulse widths, more specifically, a method in which the L level pulse width is corrected to an integer multiple of the H level pulse width before transmitted. That is, in this method, on the transmission side, the ratio of H level pulse width to L level pulse width for “0” data in the transmission waveform is set to T:2T as shown in FIG. 6A, and the ratio of H level pulse width to L level pulse width for “1” data is set to T:5T as shown in FIG. 6B. As a result of this, on the reception side, the ratio of H level pulse width to L level pulse width for “0” data and “1” data in the reception waveform become generally 1:1 and 1:3, respectively, as shown in FIGS. 6C and 6D. In this case, however, there arises a problem that pulse widths of individual transmission-side data do not meet the standard value range. That is, whereas the standard values for individual data are T=350 μs–500 μs and 3T=1050 μs–1500 μs, such large-scale corrections as correcting T to 2T and correcting 3T to 5T as shown above would cause a problem that the resulting transmission waveform deviates from the above standard value range.
As other countermeasures, there are some other methods in which correction on the reception side is performed not by determining the pulse width with a threshold set at the generally center of the reception-side waveform, but by implementing reception-side correction, for example, by determining the pulse width at the rise and fall edges of the waveform or by making the reception-side sampling interval shorter, or other means. In these methods, however, the waveform itself has been deformed by the influence of noise or the like, posing a problem of being inferior in performance to cases where the correction is performed on the transmission side.